1. Field of the Invention
This invention relates to water-based coating compositions for exterior surfaces which upon application quickly develop resistance to being washed out by inadvertent precipitation. These compositions comprise an aqueous dispersion of water-insoluble latex polymer prepared by emulsion polymerization, inorganic pigment dispersed with anionic polymers, salts or soaps and a water-soluble salt of a multivalent metal such as the propionic acid salt of calcium.
This invention relates to water-based coatings for exterior surfaces. Exterior coatings are exposed to the vicissitudes of weather from the very moment they are applied to protect an exterior surface. Water-based coatings are favored over solvent borne coatings for a number of well known reasons. They offer ease of application, reduced toxic solvent emission, lower raw material and application costs, and easy cleanup of site and equipment. However, water-based coatings are particularly vulnerable to water damage during and immediately after application. A sudden summer shower can wash away or otherwise damage the result of many hours of labor and waste valuable materials.
2. BRIEF DESCRIPTION OF THE PRIOR ART
Exterior water-based coatings may be formulated using aqueous dispersions of water-insoluble polymer latex as binders. After application, the aqueous carrier evaporates, and the individual latex particles coalesce to form an integral film. Some applications require relatively thick coating films. For example, coatings for formed-in-place polyurethane foam which is used to thermally insulate and protect institutional, commercial and industrial roofs, are relatively thick, conventionally on the order of 20-40 mils (0.05-0.10 cm). These coatings are frequently referred to as "roof mastics." Roof mastics are often applied by spray techniques which allow many acres of coating to be applied during a single work period. Because they are applied as a thick coating, they lose water by evaporation relatively slowly in comparison with other polymer latex coatings products such as house paints which are typically applied at 5-8 mils (0.013-0.02 cm). Thus, roof mastics are particularly vulnerable to being washed off by unanticipated precipitation. Typically to avoid having the roof mastic washout it is necessary to apply the full thickness in two or more separate applications which effectively doubles or triples the labor costs of the coating application. A substantial need exists for water-based roof mastics and other exterior water based coatings products which resist washout immediately after application or shortly thereafter ("early" washout resistance) and can be sprayed on vertical surfaces in any thickness without sagging or running. A water based coating that can be sprayed to any thickness on a vertical surface without sagging, running, or washout due to unanticipated precipitation could be applied in one application in the case of a roof mastic thereby eliminating the need for a second or third coat to obtain the desired thickness. This would reduce the labor costs significantly.
In U.S. Pat. No. 4,571,415, a water-soluble salt of a multivalent complex ion having labile, volatile ligands such as zinc ammonium complex ion is added to the roof mastic to make a one part composition. It is believed that early washout resistance results from multivalent metal ion precipitation of the anionic polymers used to disperse pigments and that the multivalent metal ions become available for such precipitation as the complex metal ion is shifted through the various equilibria linking it to the metal ion. The ligand, for example, is believed to be lost at the surface of the applied coating or caulk to the atmosphere. There are some problems with incorporating the flocculant into the roof mastic composition before it is applied to the roof, as in U.S. Pat. No. 4,571,415. Zinc tetramine bicarbonate requires high formulation pH (e.g., &gt;10) in order to maintain shelf stability. There also have been objections to the ammonia odor. Although the use of zinc tetramine bicarbonate speeds up the gel time of the resultant coating, by the very nature of it being used in one component system it can only speed up the coating so much before it reduces the package stability of the finished coating or mastic. The object of the present invention does away with stability problems because the preferred method of application is as a two component system. An added benefit of this invention is that the flocculating agent calcium propionate has very low toxicity and no odor as applied in preferred method of application.